Exhaust emission control system and method for internal combustion engines

ABSTRACT

An NO x  storage-and-reduction type catalyst is used which exhibits a saturated NO x  storage amount of 5 g or more as NO 2  with respect to 1 liter of a catalyst volume at 500° C., and rich spiking is controlled so that an actual NO x  storage amount of the NO x  storage-and-reduction type catalyst becomes 50% or less of the saturated NO x  storage amount. 
     Since the saturated NO x  storage amount is large, the NO x  storage amount is large even when it is 50% or less, it is possible to prolong intervals of the rich spiking. Then, since NO x  storage component stores NO x  preferentially into the sites which are likely to store and release NO x , a reduction efficiency is high. Therefore, while prolonging the intervals of the rich spiking and sustaining an effect of mileage improvement, it is possible to improve the reduction efficiency of NO x .

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying apparatus andexhaust gas purifying process for an internal combustion engine, and, inparticular, to an exhaust gas purifying apparatus and exhaust gaspurifying process which can efficiently reduce and purify NO_(x) whileinhibiting mileage from lowering.

BACKGROUND ART

Recently, the global warming phenomenon by carbon dioxide has become anissue, and to reduce the carbon dioxide emission amount has become anassignment. In automobile as well, to reduce the carbon dioxide amountin the exhaust gas has become an assignment, and a lean-burn enginewhich can thinly burn a fuel in an excessive oxygen atmosphere has beendeveloped. By this lean-burn engine, since mileage is improved, it ispossible to suppress the emission amount of carbon dioxide.

By the way, in a case where harmful components in an exhaust gas from alean-burn engine are reduced, since the exhaust gas is put in anexcessive oxygen atmosphere, reduction reactions become less likely tooccur, and the reduction and purification of NO_(x) become difficult.Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652,an NO_(x) storage-and-reduction type catalyst is disclosed on which anNO_(x) storage component, being selected from the group consisting ofalkali metals, alkaline-earth metals and rare-earth elements, is loadedalong with a noble metal. By using this NO_(x) storage-and-reductiontype catalyst and controlling an air-fuel ratio so that it becomes froma fuel-stoichiometric to rich atmosphere in a pulsating manner in themidway of a fuel-lean atmosphere, it is possible to efficiently progressthe oxidation of HC and CO as well as the reduction of NO_(x), andaccordingly it is possible to acquire high purifying performance.

Namely, an exhaust gas which is burned in a fuel-lean atmosphere becomesa reduction components lean atmosphere, in the reduction components leanatmosphere, NO in the exhaust gas is oxidized to become NO_(x) and isstored into the NO_(x) storage component, and accordingly the emissionof NO_(x) is suppressed. Then, when it is controlled from afuel-stoichiometric to rich air-fuel ratio in a pulsating manner, theexhaust gas becomes from a reduction components stoichiometric to richatmosphere. Therefore, NO_(x) are released from the NO_(x) storagecomponent, and they react with the reduction components, such as HC,which exist in the exhaust gas, so that they are reduced, andaccordingly the emission of NO_(x) is suppressed. Therefore, it ispossible to suppress the emission of NO_(x) in all of the atmospheresfrom fuel-rich to fuel-lean.

To control an air-fuel ratio in a pulsating manner so as to become froma fuel-stoichiometric to rich atmosphere is referred to as rich spiking,and the extent of making a fuel-rich atmosphere by the rich spiking isexpressed by deep or shallow. Namely, by rich spiking, making a heavydegree of fuel-rich atmosphere is referred to as “charging rich spikingdeeply,” and making a light degree of fuel-stoichiometric to richatmosphere is referred to as “charging rich spiking shallowly.” Then, inJapanese Unexamined Patent Publication (KOKAI) No. 11-107,810, and thelike, there is set forth to appropriately control the extent of the richspiking and the timing of charging it.

Then, it has been required to purify NO_(x), which are emitted from anengine being operated under a variety of fuel-lean conditions, by usingan NO_(x) storage-and-reduction type catalyst while always sustaining apurifying rate as high as 90% or more. However, in conventional NO_(x)storage-and-reduction type catalysts, when the NO_(x) amount, which isemitted from an engine in a unit period of time, is large, the richspiking should be charged at intervals of from a couple of seconds to 60seconds. However, in the excessive fuel component to be supplied as therich spiking, there exists, in addition to an amount to be used in thereduction of NO_(x), an amount to be used in controlling the combustionstate of an engine to a fuel-rich air-fuel ratio. Therefore, in a casewhere the rich spiking is charged frequently, there arises thedeterioration of mileage. In particular, in a case where being drivenordinarily at a high speed, the NO_(x) amount, which is emitted from anengine, enlarges remarkably. Therefore, in order to sustain a highNO_(x) purifying rate, the rich spiking should be kept being charged atvery short intervals of 10 seconds or less, and accordingly there is aproblem in that the mileage is lowered considerably.

The present invention has been done in view of such circumstances, andits main object is to improve mileage by prolonging the intervals of therich spiking as well as to improve the reduction-and-purificationefficiency of NO_(x).

The characteristics of an exhaust gas purifying apparatus for aninternal combustion engine, which solves the aforementioned assignments,lie in that it is used for an internal combustion engine, which canselect an operation at a fuel-lean air-fuel ratio and an operation at afuel-stoichiometric or rich air-fuel ratio and comprises: an NO_(x)storage-and-reduction type catalyst disposed in an exhaust gas flowpassage and exhibiting a saturated NO_(x) storage amount of 5 g or moreas NO₂ with respect to 1 liter of a catalyst volume at 500° C.; NO_(x)storage amount estimating means for estimating an actual NO_(x) storageamount of the NO_(x) storage-and-reduction type catalyst; air-fuel ratioadjusting means for adjusting an exhaust gas atmosphere to reductioncomponents lean or reduction components rich; and a controlling devicefor controlling the air-fuel ratio adjusting means based on an estimatedvalue estimated by the NO_(x) storage amount estimating means so thatthe actual NO_(x) storage amount becomes 50% or less of the saturatedNO_(x) storage amount.

Moreover, the characteristics of an exhaust gas purifying process for aninternal combustion engine of the present invention lie in that, in anexhaust gas purifying process for an internal combustion engine, inwhich an NO_(x) storage-and-reduction type catalyst including an NO_(x)storage component is contacted with an exhaust gas from an internalcombustion engine, which can select an operation at a fuel-lean air-fuelratio and an operation at a fuel-stoichiometric or rich air-fuel ratio,thereby storing NO_(x) contained in the exhaust gas into the NO_(x)storage component in a reduction components lean atmosphere, andreducing NO_(x) released from the NO_(x) storage component by making areduction components stoichiometric to rich atmosphere by rich spiking,an NO_(x) storage-and-reduction type catalyst exhibiting a saturatedNO_(x) storage amount of 5 g or more as NO₂ with respect to 1 liter of acatalyst volume at 500° C. is used, and in that the rich spiking iscontrolled so that an actual NO_(x) storage amount of the NO_(x)storage-and-reduction type catalyst becomes 50% or less of the saturatedNO_(x) storage amount.

The “fuel-lean air-fuel ratio” refers to an air-fuel ratio which makesan exhaust gas into an atmosphere in which oxygen exists in aconcentration which exceeds an oxygen equivalent ratio required foroxidizing all of reduction components, such as CO, THC and H₂. In thecase of an air-fuel ratio (A/F: ratio of air to fuel by weight), theequivalent (stoichiometric) point is around 14.6, and, in the presentinvention, an atmosphere whose A/F exceeds 14.6 is called a fuel-leanair-fuel ratio. Moreover, the “fuel-rich air-fuel ratio” refers,contrary to the “fuel-lean air-fuel ratio,” to an air-fuel ratio whichmakes an exhaust gas into an atmosphere in which oxygen exists in aconcentration which does not reach an oxygen equivalent ratio requiredfor oxidizing all of the reduction components, an air-fuel ratio inwhich A/F does not reach 14.6 is called a fuel-rich air-fuel ratio.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram for illustrating the difference ofNO_(x) storage sites resulting from the difference of saturated NO_(x)storage amounts.

FIG. 2 is a graph for illustrating the relationship between proportionsof actual NO_(x) storage amounts with respect to saturated NO_(x)storage amounts and NO_(x) purifying rates in an NO_(x)storage-and-reduction type catalyst which was used in Example No. 1.

FIG. 3 is a graph for illustrating the relationships between elapsingtimes of rich spiking and concentrations of emitted NO_(x).

FIG. 4 is a graph for illustrating the relationships between saturatedNO_(x) storage amounts and NO_(x) amounts which were reduced andremoved.

FIG. 5 is a block diagram of an exhaust gas purifying apparatus of anexample of the present invention.

FIG. 6 is a flow chart for illustrating the processing contents in anexhaust gas purifying apparatus of an example of the present invention.

BEST MODE FOR CARRYING OUT INVENTION

The inventors of the present application earnestly studied first therelationships between the charging timing of the rich spiking as well asits depth and the NO_(x) purifying performance by using an NO_(x)storage-and-reduction type catalyst in order to solve the aforementionedassignments. Then, as a result, they found out that the reductionefficiency was high in a case where the rich spiking was charged in astage before the NO_(x) storage amount saturates, and discovered thatthe reduction efficiency, which resulted from the rich spiking, was thehighest when the storage amount was 50% or less of the saturated NO_(x)storage amount.

However, even if the rich spiking is charged when the storage amount is50% or less of the saturated NO_(x) storage amount, if the absoluteamount of storable NO_(x) is less, the purifying rate of NO_(x) becomeslow in a case where a large amount of NO_(x) generates under thehigh-temperature fuel-lean condition, such as high-speed ordinarydriving.

Hence, it was decided to use a catalyst which could store NO_(x) in anamount equivalent to the saturated NO_(x) storage amount of aconventional NO_(x) storage-and-reduction type catalyst or more thereofeven if it exhibited the storage amount of 50% or less of the saturatedNO_(x) storage amount. The present invention has been done by suchdiscoveries and the selection of an optimum catalyst.

Note that the “saturated NO_(x) storage amount” refers to a total NO_(x)amount which is stored in a catalyst, after NO_(x) stored in thecatalyst is fully reduced, between the time of beginning theintroduction of NO_(x) into the catalyst and the time when thecatalyst-outlet-gas NO_(x) concentration reaches the catalyst-inlet-gasNO_(x) concentration. In the present invention, this value is found byconverting the NO_(x) amount stored with respect to 1 liter of thecatalyst into the NO₂ weight.

Namely, in the exhaust gas purifying apparatus and exhaust gas purifyingprocess of the present invention, an NO_(x) storage-and-reduction typecatalyst whose saturated NO_(x) storage amount is 5 g or more as NO₂ at500° C. is used, and the rich spiking is controlled so that the actualNO_(x) storage amount of the NO_(x) storage-and-reduction type catalystbecomes 50% or less of the saturated NO_(x) storage amount. Thus, evenwhen the frequency of the rich spiking is made equal to conventional oneor longer, it is possible to reduce most of the stored NO_(x), and thereduction efficiency is improved greatly. Therefore, it is possible tosatisfy both of the improvement of mileage and the improvement of NO_(x)purifying rate.

Note that the upper limit value of the saturated NO_(x) storage amountis defined by the amount of used NO_(x) storage component, however, in acase where barium is used as an NO_(x) storage component (2 mol asbarium carbonate), 184 g with respect to 1 liter of the catalyst is theupper limit value. This is because, even when barium is loaded more thanthis, the effect saturates, and the activity lowers since the loadednoble metal is covered with barium.

As the NO_(x) storage-and-reduction type catalyst whose saturated NO_(x)storage amount is 5 g or more as NO₂ at 500° C., as set forth inJapanese Unexamined Patent Publication (KOKAI) No. 10-249,199, it ispossible to use one which is composed of a support comprising acomposite oxide expressed by MgO—Al₂O₃, which is prepared by a sol-gelmethod with a magnesium salt and an aluminum alkoxide as startingmaterials, and at least one NO_(x) storage component as well as a noblemetal, which are loaded on the support, the NO_(x) storage componentbeing selected from the group consisting of alkali metals,alkaline-earth metals and rare-earth elements.

Further, it is possible to use an NO_(x) storage-and-reduction typecatalyst which uses a support comprising alumina particles and acomposite oxide layer having a structure which is formed on at leastpart of the surface of the alumina particles and which is expressed by achemical formula, MO—nAl₂O₃ (“M” is at least one member selected fromthe group consisting of alkaline-earth metals and rare-earth elements.).

Furthermore, it is possible to use an NO_(x) storage-and-reduction typecatalyst which uses a support comprising a first composite oxide,expressed by MgO—Al₂O₃, and a second composite oxide, expressed byTiO₂—ZrO₂.

The composite oxide, such as MgO—Al₂O₃, is a spinel compound, since itexhibits a higher basicity than alumina does, the NO_(x) storage abilityin a high-temperature region improves. Therefore, when such a support isused, it is possible to make an NO_(x) storage-and-reduction typecatalyst which exhibits a saturated NO_(x) storage amount of 5 g or moreas NO₂ in the high-temperature region like 500° C.

Note that, when the saturated NO_(x) storage amount is less than 5 g asNO₂ at 500° C., the intervals between the rich spiking should beshortened in order to keep using so as to be the NO_(x) storage amountof 50% or less. In this case, since, among the excessive fuel componentswhich are supplied as the rich spiking, the amount, which is used inorder to control the combustion state of an engine to a fuel-richair-fuel ratio, is increased so that the amount used for the reductionof NO_(x) decreases, the effect of the mileage improvement is hardlyrevealed.

On the aforementioned support, an NO_(x) storage component and a noblemetal are loaded to form an NO_(x) storage-and-reduction type catalyst.As the NO_(x) storage component, an element is used which is selectedfrom the group consisting of alkali metals, such as K, Na, Li and Cs,alkaline-earth metals, such as Ba, Ca, Sr and Mg, or rare-earthelements, such as La, Sc and Y. Moreover, as the noble metal, Pt, Rh,Pd, Ir, and the like, are exemplified. The loading amount of the NO_(x)storage component can desirably fall in a range of from 0.4 to 2.0 molein the total amount with respect to 1 liter of the support, and theloading amount of the noble metal can desirably fall in a range of from2 to 20 g with respect to 1 liter of the support.

In the exhaust gas purifying process of the present invention, theaforementioned NO_(x) storage-and-reduction type catalyst is contactedwith an exhaust gas from an internal combustion engine, which can selectan operation at a fuel-lean air-fuel ratio and an operation at afuel-stoichiometric or rich air-fuel ratio. Since the exhaust gas, whichis burned with a fuel-lean air-fuel ratio, is turned into a reductioncomponents lean atmosphere, NO in the exhaust gas is oxidized on thecatalyst to turn into NO_(x), and is stored in the NO_(x) storagecomponent on the catalyst. Then, when the exhaust gas is turned into areduction components rich atmosphere by charging the rich spiking,NO_(x), which have been stored in the NO_(x) storage component, arereleased, and are reduced by the reduction components, such as CO andHC, in the exhaust gas.

Then, in the present invention, the NO_(x) storage-and-reduction typecatalyst is used whose saturated NO_(x) storage amount is 5 g or more asNO₂ at 500° C., and the rich spiking is controlled so that the actualNO_(x) storage amount of the NO_(x) storage-and-reduction type catalystbecomes 50% or less, more desirably 30% less, of the saturated NO_(x)storage amount. Namely, the rich spiking is charged in a state that theactual NO_(x) storage component is 2.5 g or less, more desirably 1.5 gor less. By thus charging the rich spiking in a state that does notsatisfy the saturated NO_(x) storage amount, the reduction efficiency ofNO_(x) is heightened extremely, and it is possible reduce and purifymost of the stored NO_(x).

The reason why it is thus effected is not clear, however, it is assumedas follows. Namely, it is believed that, in the NO_(x) storage sites ofa catalyst, a variety of sites exist from sites, which are less likelyto store NO_(x) and are less likely to release them, to sites, whichstore NO_(x) with ease relatively and are likely to release them.Suppose that the distribution of sites is uniform, and let us consider acase where NO_(x) are stored in an equal amount in a catalyst “A” havinga greater saturated NO_(x) storage ability and a catalyst “B” having aless saturated NO_(x) storage ability, respectively.

In FIG. 1, the saturated NO_(x) storage abilities correspond to theareas of the rectangles, the rectangle having a larger area illustratesthe catalyst “A” having a greater saturated NO_(x) storage ability, andthe rectangle having a smaller area illustrates the catalyst “B” havinga less saturated NO_(x) storage ability. The hatched portions having thesame area show the stored NO_(x) amount. As illustrated in FIG. 1, inthe catalyst “A,” NO_(x) are stored in sites which store NO_(x) withease and are likely to release them, but, in the catalyst “B,” NO_(x)are loaded on up to sites which are less likely store NO_(x) and areless likely to release them. In such states, when the rich spiking ischarged, it is believed that the reduction efficiency is high becauseNO_(x) are readily released from the catalyst “A” and are reduced, andthat, on the other hand, the reduction efficiency becomes low becauseNO_(x) are less likely to release from the catalyst “B.”

Moreover, since the saturated NO_(x) storage amount of the NO_(x)storage-and-reduction type catalyst is large in the present invention,even in a case where rich spiking is charged so that the NO_(x) storageamount is 50% or less, or 30% or less, it is possible to make theintervals between the rich spiking equal to the conventional ones orlonger. Therefore, it is possible to avoid the drawback of loweringmileage, from this sense as well, the timing of charging the richspiking can preferably be carried out, although it depends on thesaturated NO_(x) storage amount, at the moment when the NO_(x) storageamount is 50% or less, or 30% or less, or at the moment when it reachesan amount as adjacent as possible to 50%, or 30%.

The exhaust gas purifying apparatus of the present invention, which cansecurely carry out the aforementioned exhaust gas purifying process ofthe present invention, is constituted by an NO_(x) storage-and-reductiontype catalyst, NO_(x) storage amount estimating means, air-fuel ratioadjusting means and a controlling device.

It is possible to use for the NO_(x) storage-and-reduction typecatalyst, which is the same one as used in the aforementioned exhaustgas purifying process of the present invention, and one, which exhibitsa saturated NO_(x) storage amount of 5 g or more as NO₂ with respect to1 liter of a catalyst volume at 500° C., is used.

The NO_(x) storage amount estimating means is means, which estimates anactual NO_(x) storage amount of the NO_(x) storage-and-reduction typecatalyst. The NO_(x) amount, which is stored and held by the NO_(x)storage-and-reduction type catalyst, is an NO_(x) amount, which isstored in the NO_(x) storage-and-reduction type catalyst in a unitperiod of time, and is proportional to an NO_(x) amount, which isgenerated at an engine in a unit period of time. While, an NO_(x)amount, which is generated at an engine in a unit period time, isdecided by a fuel supply amount to an engine, an air-fuel ratio, anexhaust flow rate, and the like, and accordingly, when the runningconditions of an engine are determined, it is possible to know an NO_(x)amount, which is stored in the NO_(x) storage-and-reduction typecatalyst. Moreover, the estimation of actual NO_(x) storage amount canbe carried out by calculating from the fluctuating circumstances in thenumber of engine revolutions or the temperatures of exhaust gases, or itis possible to carry it out by measuring the NO_(x) amounts incatalyst-inlet gases.

The air-fuel ratio adjusting means is means, which adjusts an exhaustgas atmosphere to reduction components lean or reduction components richby adjusting an air-fuel ratio to fuel lean or fuel rich, and varies theatmospheres of exhaust gases by varying a fuel injection timing, asuction air amount, an inlet air pressure, a fuel supply amount, and thelike.

The control device is a device, which controls, based on an estimatedvalue estimated by the NO_(x) storage amount estimating means, theair-fuel ratio adjusting means so that the actual NO_(x) storage amountbecomes 50% or less of the saturated NO_(x) storage amount, and acomputer is used for it.

To control the rich spiking so that it is charged when the NO_(x)storage amount is 50% or less, or 30% or less, or at the moment when itreaches an amount as adjacent as possible to 50%, or 30%, it is possibleto carry it out by estimating an accumulated NO_(x) amount, which hasbeen stored in the NO_(x) storage-and-reduction type catalyst while theengine is running at a fuel-lean air-fuel ratio, with the NO_(x) storageamount estimating means, and, when the accumulated NO_(x) amount shows adesignated value being 50% or less, or 30% less, of a saturated NO_(x)amount for an NO_(x) storage-and-reduction type catalyst, which has beenknown in advance, by controlling the fuel injection timing, the suctionair amount and the fuel injection amount with the air-fuel ratioadjusting means so as to switch from the fuel-lean air-fuel ratio to afuel-rich air-fuel ratio in a short period of time.

Namely, in accordance with the exhaust gas purifying process of thepresent invention, it is possible to efficiently reduce and purifyNO_(x) while inhibiting the mileage from lowering by prolonging theintervals between the rich spiking. Moreover, in accordance with theexhaust gas purifying apparatus of the present invention, it is possibleto securely carry out the exhaust gas purifying process of the presentinvention.

EXAMPLES

Hereinafter, the present invention will be described in detail withreference to examples and a comparative example.

(Example No. 1)

38 parts by weight of magnesium acetate tetrahydrate, 72 parts by weightof aluminum isopropoxide (Al[OCH(CH₃)₂]₃) and 400 parts by weight ofisopropyl alcohol were mixed (Mg:Al=1:2 by molar ratio), and wererefluxed at 80° C. for about 2 hours while stirring them. 60 parts byweight of ion-exchange water was dropped thereto to complete hydrolysis,were further kept being refluxed at 80° C. for 2 hours, and werethereafter cooled.

Subsequently, the solvents were removed on a water bath by using arotary evaporator, were further dried naturally at room temperature for24 hours, and were thereafter calcined in air at 850° C. for 5 hours,thereby obtaining a composite oxide support powder having a compositionof MgO—Al₂O₃.

This support powder was made into a slurry, and, by using an alumina solas a binder, a coating layer was formed on a honeycomb substrate(diameter: 103 mm and length: 150 mm), which was made from cordierite,by an ordinary method. The coating layer was formed in an amount of 240g with respect to 1 liter of the honeycomb substrate.

Subsequently, into the honeycomb substrate with the coating layer, adiammine dinitro platinum (II) solution, which had a predeterminedconcentration, was impregnated in a predetermined amount, was evaporatedand dried to solidify, and was thereafter calcined in air at 300° C. for3 hours to load Pt. The loading amount of Pt was 10 g with respect to 1liter of the honeycomb substrate.

Subsequently, into the honeycomb substrate with Pt loaded, a potassiumacetate aqueous solution, which had a predetermined concentration, wasimpregnated in a predetermined amount, was evaporated and dried tosolidify, and was thereafter calcined in air at 300° C. for 3 hours toload K. The loading amount of K was 0.0.6 mol with respect to 1 liter ofthe honeycomb substrate.

A saturated NO_(x) storage amount of the resulting NO_(x)storage-and-reduction type catalyst of Example No. 1 was 17 g as NO₂ at500° C.

This NO_(x) storage-and-reduction type catalyst of Example No. 1 wasinstalled in an exhaust system of an in-line 4-cylinder 2-Ldirect-injection engine, and NO_(x) purifying rates were measured underthe evaluation conditions of the number of engine revolutions: 2,000rpm, torque: 60 Nm, and catalyst-inlet temperature: 500° C. in thefollowing manner.

The NO_(x) storage-and-reduction type catalyst was used which wasreduced completely by exposing it to an atmosphere, which was aftersetting the A/F of the engine, running under the aforementionedconditions, to 12, for 10 minutes. Then, the engine was operated under afuel-lean atmosphere of A/F=20 under the aforementioned conditions, andthe rich spiking of A/F=12 was charged every 5 minutes for 2 seconds. Inthis instance, the NO_(x) purifying rates and NO_(x) storage amountsafter 1 minute from the charging of the rich spiking were measured.Then, the ratios of the actual NO_(x) storage amounts with respect tothe saturated NO_(x) storage amounts were measured, and the relationshipbetween the values and NO_(x) purifying ratios is illustrated in FIG. 2.

From FIG. 2, it is seen that, in a case where the proportion of theactual NO_(x) storage amount with respect to the saturated NO_(x)storage amount was 50% or less, the NO_(x) purifying ratio become 80% ormore, and that, in a case where the proportion of the actual NO_(x)storage amount with respect to the saturated NO_(x) storage amount was30% or less, the NO_(x) purifying ratio become 90% or more. Namely, whenan NO_(x) storage-and-reduction type catalyst of 5 g/L is used, it isnot necessary to charge the rich spiking until NO_(x) were stored in anamount of 30% thereof, i.e., up to 1.5 g/L. Thus, it is possible to makethe sufficient NO_(x) purification and the mileage improvementcompatible.

(Comparative Catalyst)

120 g of an Al₂O₃ powder, 120 g of a TiO₂ powder, 50 g of a ZrO₂ powderand 20 g of a CeO₂—ZrO, powder were mixed, were made into a slurry, and,by using an alumina sol as a binder, a coating layer was formed on ahoneycomb substrate (diameter: 103 mm and length: 150 mm), which wasmade from cordierite, by an ordinary method. The coating layer wasformed in an amount of 270 g with respect to 1 liter of the honeycombsubstrate.

Subsequently, into the honeycomb substrate, a barium acetate aqueoussolution, which had a predetermined concentration, was impregnated in apredetermined amount, was evaporated and dried to solidify, and wasthereafter calcined in air at 300° C. for 3 hours to load Ba.Thereafter, it was immersed into a solution, which included ammoniumcarbonate in an amount as much as 3 times of Ba, for 1 hour to turn Bainto carbonate, and it was calcined at 300° C. for 3 hours.

Subsequently, into the honeycomb substrate having the coating layer withBa loaded, a diammine dinitro platinum (II) solution, which had apredetermined concentration, was impregnated in a predetermined amount,was evaporated and dried to solidify, and was thereafter calcined in airat 300° C. for 3 hours to load Pt. Subsequently, a rhodium nitrateaqueous solution, which had a predetermined concentration, wasimpregnated in a predetermined amount, was evaporated and dried tosolidify, and was thereafter calcined in air at 300° C. for 3 hours toload Rh. The loading amounts of Pt and Rh were 2.0 g for Pt and 0.5 gfor Rh with respect to 1 liter of the honeycomb substrate. Subsequently,a potassium acetate aqueous solution, which had a predeterminedconcentration, was impregnated in a predetermined amount, was evaporatedand dried to solidify, and was thereafter calcined in air at 300° C. for3 hours to load K. The loading amounts of Ba and K were 0.2 mol for Baand 0.1 mol for K with respect to 1 liter of the honeycomb substrate.

A saturated NO_(x) storage amount of the resulting NO_(x)storage-and-reduction type catalyst was 2 g as NO₂ at 500° C. ThisNO_(x) storage-and-reduction type catalyst was considered a comparativecatalyst.

The catalysts of Example No. 1 and the aforementioned comparativeexample were installed, respectively, in an exhaust system of an in-line4-cylinder 2-L direct-injection engine, and NO_(x) conversions weremeasured under the evaluation conditions of the number of enginerevolutions: 2,000 rpm, torque: 60 Nm, and catalyst-inlet temperature:500° C. in the following manner.

The catalyst of Example No. 1 and the aforementioned comparativecatalyst were used which were reduced completely by treating them in ahydrogen gas flow at 500° C. for 3 hours. Then, the engine was operatedunder a fuel-lean atmosphere of A/F=20 under the aforementionedconditions, and the rich spiking of A/F=12 was charged for 1 second. Thecharging intervals of the rich spiking were put at 4 levels, 30, 60, 90and 120 seconds, and the NO_(x) conversions were measured in the initialrich-spiking-charging section at 10 minutes after starting theoperation. The results are set forth in Table 1.

-   -   subsequently, in a case where the charging of the rich spiking        was carried out at intervals of 90 seconds, the elapsed times of        fuel-rich atmosphere and the NO_(x) concentrations in the outlet        gases at the elapsed times were measured, respectively, and the        results are illustrated in FIG. 3. In FIG. 3, the NO_(x)        concentration in the inlet gases is illustrated as well.

TABLE 1 NO_(x) Conversion (%) Rich-Spiking Catalyst of ComparativeInterval Ex. No. 1 Catalyst 30 sec. 99.8 95.3 60 sec. 99.5 91.3 90 sec.99.0 82.2 120 sec.  92.5 66.8

From Table 1, in the case of the NO_(x) storage-and-reduction typecatalyst of Example No. 1, it is seen that the high NO_(x) conversionsof 90% or more were exhibited under all of the conditions. However, inthe case of the comparative catalyst, when the rich spiking was chargedat intervals of up to 60 seconds, the NO_(x) conversions of 90% or morewere exhibited, but, when the intervals become longer than that, theNO_(x) conversions were lowered.

Namely, in accordance with the NO_(x) storage-and-reduction typecatalyst of Example No. 1, even if the intervals of the rich spikingwere long, a high NO_(x) reduction efficiency was exhibited, and it isbelieved that this resulted from the fact that the saturated NO_(x)storage amount was as high as 17 g.

Moreover, from FIG. 3, it is apparent that, in the comparative catalyst,although the NO_(x) emission concentration was lowered sharply after therich spiking, the NO_(x) emission concentration was increased so thatthe NO_(x) reduction efficiency was lowered, on the other hand, in theNO_(x) storage-and-reduction type catalyst of Example No. 1, theemission of NO_(x) was substantially zero during the rich spiking sothat the reduction efficiency was remarkably high.

(Example No. 2)

Note that a variety of NO_(x) storage-and-reduction type catalysts,whose saturated NO_(x) storage amounts were different, were prepared inthe same manner as Example No. 1, and the NO_(x) purifying amounts at 1minute from the charging of the rich spiking were measured in the samemanner as Example No. 1. The results are illustrated in FIG. 4. TheNO_(x) purifying amounts are expressed by weights which are convertedinto NO₂.

Conventionally, since it has been considered that the saturated NO_(x)storage amount and the reduction efficiency are unrelated, if such isthe case, the 1-minute NO_(x) purifying amount should be constant, asillustrated by the hatched line of FIG. 4, regardless of the saturatedNO_(x) storage amount. However, as a result of the aforementioned test,it has been evident that, as illustrated by the solid line of FIG. 4,the 1-minute NO_(x) purifying amount is enlarged as the saturated NO_(x)storage amount is increased.

(Example No. 3)

In FIG. 5, a construction of an exhaust gas purifying apparatus of thisexample is illustrated. This exhaust gas purifying apparatus isconstituted by an NO_(x) storage-and-reduction type catalyst 1, which isdisposed in an exhaust gas flow passage from an automobile internalcombustion engine and which exhibits a saturated NO_(x) storage amountof 5 g or more as NO₂ with respect to 1 liter of a catalyst volume at500° C., NO_(x) storage amount estimating means 2for estimating anactual NO_(x) storage amount of the NO_(x) storage-and-reduction typecatalyst 1, air-fuel ratio adjusting means 3 for adjusting an exhaustgas atmosphere to reduction components lean or reduction components richby controlling an air-fuel ratio to fuel lean or fuel rich, and acontrolling device 4 for controlling the air-fuel ratio adjusting means3 based on an estimated value estimated by the NO_(x) storage amountestimating means 2 so that the actual NO_(x) storage amount becomes 50%or less of the saturated NO_(x) storage amount.

The NO_(x) storage amount estimating means 2 and controlling device 4are constituted by a computer (ECU). Moreover, on cases where the engineoperation conditions (the accelerator opening extent, the number ofengine revolutions, the suction air amount, the inlet pressure, theair-fuel ratio, the fuel supply amount, and the like) are varied, theamounts of NO_(x), which are generated from the engine in a unit periodof time, are measured actually, respectively. Then, in a RON of the ECU,the amounts of NO_(x), which are stored in the NO_(x)storage-and-reduction type catalyst 1 in a unit period of time, aresaved as a form of a numerical map in which the engine loads (fuelinjection amounts) and the number of engine revolutions are used.

Moreover, the air-fuel ratio adjusting means 3 is mainly composed of afuel injection device, and varies the exhaust gas atmosphere by varyingthe fuel injection timing, the suction air amount, the inlet pressure,the fuel supply amount, and so on, with the control device 4.

Hereinafter, the operation of the exhaust gas purifying a apparatus ofthis example will be described with reference to the flow chartillustrated in FIG. 6.

First, at step 10, the engine is operated with a fuel-lean air-fuelratio. At step 11, the ECU calculates at every predetermined time anNO_(x) amount (Nt), which is stored in the NO_(x) storage-and-reductiontype catalyst 1 in a unit period of time, from the engine load (fuelinjection amount) and the number of engine revolutions by using the mapin the ROM. Then, at step 12, an NO_(x) counter “N” is increased by thisNO_(x) storage amount “Nt.” Thus, a value of the NO_(x) counter “N”comes to always specify an amount of NO_(x) which are stored in theNO_(x) storage-and-reduction type catalyst 1.

Then, the ECU does not do anything, at step 13, in a case where thevalue of the aforementioned NO_(x) counter “N” does not exceed a setvalue “S,” which has been set in advance, and accordingly the fuel-leanair-fuel ratio operation is maintained. Then, at step 13, in a casewhere the value of the aforementioned NO_(x) counter “N” is increased tothe set value “S,” which has been set in advance, or more, at step 14,the air-fuel ratio adjusting means 3 is controlled to switch to afuel-rich air-fuel ratio operation so that the exhaust gas atmosphere ofthe engine is varied to reduction components rich. This rich spiking iscarried out only for a predetermined short period of time by a timer atstep 15. Thus, since a reduction components-rich exhaust gas is flowedinto the NO_(x) storage-and-reduction type catalyst 1, NO_(x), whichhave been stored in the NO_(x) storage-and-reduction type catalyst 1,are released so that they are reduced and purified.

Then, after the rich spiking is completed for the predetermined time,the counter “N” is reset to an initial value at step 16, and theoperation returns back again to step 10 so that the air-fuel ratioadjusting means 3 is controlled to switch to a fuel-lean air-fuel ratiooperation.

Therefore, it is possible to securely carry out the exhaust gaspurifying process of the present invention by using the exhaust gaspurifying apparatus of this example, by using such a catalyst, used inExample No. 1, etc., that exhibits a saturated NO_(x) storage amount of5 g or more as NO₂ with respect to 1 liter of a catalyst volume at 500°C., and by setting the set value “S” to a value of 50% or less, or 30%or less, of the saturated NO_(x) storage amount.

1. An exhaust gas purifying apparatus used for an internal combustionengine, which can select an operation at a fuel-lean air-fuel ratio andan operation at a fuel-stoichiometric or rich air-fuel ratio, and whichapparatus comprises: an NO_(x) storage-and-reduction type catalystdisposed in an exhaust gas flow passage and exhibiting a saturatedNO_(x) storage amount of 5 g or more as NO₂ with respect to 1 liter of acatalyst volume at 500° C.; NO_(x) storage amount estimating means forestimating an actual NO_(x) storage amount of the NO_(x)storage-and-reduction type catalyst; air-fuel ratio adjusting means foradjusting an exhaust gas atmosphere to reduction components lean orreduction components rich; and a controlling device for controlling theair-fuel ratio adjusting means based on an estimated value estimated bythe NO_(x) storage amount estimating means so that the actual NO_(x)storage amount becomes 50% or less of the saturated NO_(x) storageamount.
 2. The exhaust gas purifying apparatus, set forth in claim 1,characterized in that said controlling device controls said air-fuelratio adjusting means so that the actual NO_(x) storage amount becomes30% or less of the saturated NO_(x) storage amount of said NO_(x)storage-and-reduction type catalyst.
 3. The exhaust gas purifyingapparatus, set forth in claim 1, characterized in that said NO_(x)storage-and-reduction type catalyst comprises a support comprised of anMgO—Al₂O₃ composite oxide.
 4. In an exhaust gas purifying process for aninternal combustion engine, in which an NO_(x) storage-and-reductiontype catalyst including an NO_(x) storage component is contacted with anexhaust gas from an internal combustion engine, which can select anoperation at a fuel-lean air-fuel ratio and an operation at afuel-stoichiometric or rich air-fuel ratio, thereby storing NO_(x)contained in the exhaust gas into the NO_(x) storage component in areduction components lean atmosphere, and reducing NO_(x) released fromthe NO_(x) storage component by making a reduction componentsstoichiometric to rich atmosphere by rich spiking, the exhaust gaspurifying process characterized in that: an NO_(x) storage-and-reductiontype catalyst exhibiting a saturated NO_(x) storage amount of 5 g ormore as NO₂ with respect to 1 liter of a catalyst volume at 500° C. isused, and the rich spiking is controlled so that an actual NO_(x)storage amount of the NO_(x) storage-and-reduction type catalyst becomes50% or less of the saturated NO_(x) storage amount.
 5. The exhaust gaspurifying process, set forth in claim 4, characterized in that the richspiking is controlled so that the actual NO_(x) storage amount of saidNO_(x) storage-and-reduction type catalyst becomes 30% or less of thesaturated NO_(x) storage.
 6. The exhaust gas purifying process, setforth in claim 4, characterized in that said NO_(x)storage-and-reduction type catalyst comprises a support comprised of anMgO—Al₂O₃ composite oxide.
 7. The exhaust gas purifying process, setforth in claim 4, characterized in that the NO_(x) storage-and-reductiontype catalyst exhibits a saturated NO_(x) storage amount of 10 g or moreas NO₂ with respect to 1 liter of a catalyst volume at 500° C.
 8. Theexhaust gas purifying process, set forth in claim 7, characterized inthat the rich spiking is controlled so that the actual NO_(x) storageamount of said NO_(x) storage-and-reduction type catalyst becomes 30% orless of the saturated NO_(x) storage amount.
 9. The exhaust gaspurifying process, set forth in claim 4, characterized in that theNO_(x) storage-and-reduction type catalyst exhibits a saturated NO_(x)storage amount of 15 g or more as NO₂ with respect to 1 liter of acatalyst volume at 500° C.
 10. The exhaust gas purifying process, setforth in claim 9, characterized in that the rich spiking is controlledso that the actual NO_(x) storage amount of said NO_(x)storage-and-reduction type catalyst becomes 30% or less of the saturatedNO_(x) storage amount.